A microprocessor device is a central processing unit or CPU for a digital processor which is usually contained in a single semiconductor integrated circuit or "chip" fabricated by MOS/LSI technology, as shown in U.S. Pat. No. 3,757,306 issued to Gary W. Boone and assigned to Texas Instruments Incorporated. The Boone patent shows a single-chip 8-bit CPU including a parallel ALU, registers for data and addresses, an instruction register and a control decoder, all interconnected using the von Neumann architecture and employing a bidirectional parallel bus for data, address and instructions. U.S. Pat. No. 4,074,351, issued to Gary W. Boone, and Michael J. Cochran, assigned to Texas Instruments Incorporated, shows a single-chip "microcomputer" type device which contains a 4-bit parallel ALU and its control circuitry, with on-chip ROM for program storage and on-chip RAM for data storage, constructed in the Harvard architecture. The term microprocessor usually refers to a device employing external memory for program and data storage, while the term microcomputer refers to a device with on-chip ROM and RAM for program and data storage. In describing the instant invention, the term "microcomputer" will be used to include both types of devices, and the term "microprocessor" will be primarily used to refer to microcomputers without on-chip ROM; since the terms are often used interchangeably in the art, however, it should be understood that the use of one or the other of these terms in this description should not be considered as restrictive as to the features of this invention.
Modern microcomputers can be grouped into two general classes, namely general-purpose microprocessors and special-purpose microcomputers/microprocessors. General purpose microprocessors, such as the M68020 manufactured by Motorola, Inc., are designed to be programmable by the user to perform any of a wide range of tasks, and are therefore often used as the central processing unit in equipment such as personal computers. Such general-purpose microprocessors, while having good performance for a wide range of arithmetic and logical functions, are of course not specifically designed for or adapted to any particular one of such functions. In contrast, special-purpose microcomputers are designed to provide performance improvement for specific predetermined arithmetic and logical functions for which the user intends to use the microcomputer. By knowing the primary function of the microcomputer, the designer can structure the microcomputer in such a manner that the performance of the specific function by the special-purpose microcomputer greatly exceeds the performance of the same function by the general-purpose microprocessor regardless of the program created by the user.
One such function which can be performed by a special-purpose microcomputer at a greatly improved rate is digital signal processing, specifically the computations required for the implementation of digital filters and for performing Fast Fourier Transforms. Because such computations consist to a large degree of repetitive operations such as integer multiply, multiple-bit shift, and multiply-and-add, a special-purpose microcomputer can be constructed specifically adapted to these repetitive functions. Such a special-purpose microcomputer is described in U.S. Pat. No. 4,577,282, assigned to Texas Instruments Incorporated. The specific design of a microcomputer for these computations has resulted in sufficient performance improvement over general purpose microprocessors to allow the use of such special-purpose microcomputers in real-time applications, such as speech and image processing.
Digital signal processing applications, because of their computation intensive nature, also are rather intensive in memory access operations. Accordingly, the overall performance of the microcomputer in performing a digital signal processing function is not only determined by the number of specific computations performed per unit time, but also by the speed at which the microcomputer can retrieve data from, and store data to, system memory. Prior special-purpose microcomputers, such as the one described in said U.S. Pat. No. 4,577,282, have utilized modified versions of a Harvard architecture, so that the access to data memory may be made independent from, and simultaneous with, the access of program memory. Such architecture has, of course, provided for additional performance improvement.
In digital signal processing applications such as digital filters, as well as in other applications such as data acquisition, it is useful to maintain a block of memory reserved for data which is to be repetitively accessed and updated. A circular addressing sequence accesses memory locations in sequence in the block, and "wraps around" upon reaching the bottom of the block so that the top of the block is the next location to be accessed in the sequence. If the circular addressing operation is performed in software, the calculation of each memory address to be accessed requires a comparison of the calculated address against the block limits, and conditional modification of the address depending upon its relationship to the limits of the reserved block. Such decision-making, if done by the primary arithmetic logic unit of the microcomputer, adds undesired overhead to the performance of the application which uses circular addressing, in effect reducing the performance advantages gained by using a circular addressing scheme.
It is therefore an object of this invention to provide a microcomputer having an auxiliary arithmetic logic unit for the computation of memory addressing, this arithmetic logic unit capable of operating in parallel with the central processing unit of the microcomputer.
It is another object of this invention to provide such an auxiliary arithmetic logic unit capable of efficiently performing the computations necessary in circular addressing.
It is another object of this invention to provide such a microcomputer having two such auxiliary arithmetic logic units operating in parallel.
It is another object of this invention to provide such a microcomputer where the circular addressing computations may be performed either prior to or after the memory access is performed.
Further objects and advantages of the instant invention will become apparent to those of ordinary skill in the art having reference to the following specification, together with its drawings.